1. Field of the Invention
The present invention relates generally to apparatus for producing thin uniform films or layers, such as magnetic thin films, on a substrate by sputtering. More particularly, the present invention relates to a facing target type of sputtering apparatus. In a facing target type of sputtering apparatus, at least a pair of target planes are arranged to face each other in a vacuum vessel, and magnetic fields are generated perpendicularly to the target planes for confining plasma in the space between the facing target planes. The substrate is arranged so as to be positioned at the side of the space so that films are produced on the substrate by sputtering.
2. Description of the Related Art
A typical facing target type of sputtering apparatus as shown in FIG. 1 is disclosed in Japanese Examined Patent Publication No. 63-20303, 63-20304 and 62-14633 applied by the present inventor and others includes a vacuum vessel 10 for defining therein a confined vacuum chamber, an air exhausting unit (not shown) having a vacuum pump system to cause a vacuum via an outlet 30, and a gas supplying unit (not shown) for introducing a preselected sputtering gas into the vacuum vessel via an inlet 40 so that the introduced gas pressure within the vacuum vessel is maintained at a predetermined level. A pair of target portions 100a and 100b are arranged to be in the vacuum vessel 10 in such a manner that a pair of rectangular shape cathode targets 110a and 110b face each other so as to define a predetermined space therebetween. The substrate holder 21 supports a substrate 20 at a position beside the pace 120 extending the outlet of the facing targets 110a and 110b, and is arranged to be perpendicular to the planes of the targets 110a and 110b. The targets 110a and 110b are electrically connected to a later-described electric power source 50 to cause sputtering from the surfaces of targets 110a and 110b. And magnetic fields are generated to cover vertically the outside of the space between facing target planes by the arrangement of magnets 130a and 130b installed in touch with the backside planes of facing targets 110a and 110b. Shield plates 140a and 140b are used for maintaining sputtering plasma stably in a defined space between facing target planes while sputtering.
As illustrated in FIG. 1, the facing targets 110a and 110b to be a cathode, and the shield plates 140a and 140b to be the anode (the earth) are electrically connected to the output terminals of a direct current (DC) power source 50. The vacuum vessel 10 and the shield plates 140a and 140b are electrically connected to the anode, too. Under a predetermined pressure level of approximately 10.sup.-2 .about.10 Pa, sputtering plasma is formed in the space 120 between the facing targets 110a and 110b while applying an electric power from the power source 50. Since magnetic fields are generated around the peripheral area extending in a direction perpendicular to the surfaces of facing targets 110a and 110b, highly energized electrons sputtered from surfaces of the facing targets are confined in the space 120 between facing targets 110a and 110b to cause increased ionized gases by collision in the space. The ionization rate of the sputtering gases corresponds to the deposition rate of thin films on the substrate 20, then, high rate deposition is realized due to the confinement of electrons in the space 120 between the facing targets 110a and 110b. The substrate 20 is arranged so as to be isolated from the plasma space 120 between the facing targets 110a and 110b. Film deposition on the substrate is processed at a low temperature range due to a very small number of impingement of plasma from the plasma space and small amount of thermal radiation from the target planes. A typical facing target type of sputtering method has superior properties of depositing ferromagnetic materials at high rate deposition and low substrate temperature in comparison with a magnetron sputtering method. Facing target types of sputtering apparatus are used mainly for the production of films of magnetic materials, magnetic recording layers and magnetic heads due to superior confinement of electrons between facing target space.
However, when the circular or rectangular types of target are usually used, the central areas of facing targets are locally and concentrically eroded. Therefore, the efficiency in use of the sputtering surface of the facing targets is low. In addition, an uneven thickness of the film occurs in the direction of the width of a substrate, and accordingly, the conventional facing target type of sputtering apparatus is not suitable for a large-scale production of films.
To overcome these problems encountered by the above-described facing target type of sputtering apparatus, the present inventor and others proposed a different facing target type of sputtering apparatus in Japanese Examined Patent Publications No.H3-2231 and S63-54789. The proposed facing target type of sputtering apparatus has a construction such that magnetic core members forming a part of a magnetic field generating unit are arranged to be installed in the outer space around the facing targets. Thus, magnetic fields are concentratedly generated in the region surrounding the facing targets and the space therebetween. Therefore, the distribution of the magnetic fields is not affected by the magnetic permeability and saturated magnetization of the materials of facing targets or the thickness of the targets, and thus is always stable. Furthermore, since the magnetic field for capturing or confining plasma is generated around the outside area surrounded by the facing targets, the eroded area of target surfaces extends from the central portion toward the periphery of target surfaces. Accordingly, the efficiency in use of targets is remarkably improved. However, a high range of sputtering gas pressure is needed for sputtering which results in degraded quality of thin films deposited on such substrates. Moreover, the present inventor and others proposed another type of FTS (Facing Target Type Sputtering) apparatus in Japanese Examined Patent Publications No.H4-11624 and H5-75827 (U.S. Pat. No. 4,784,739) in order to generate the more homogenous distribution of plasma confinement near the whole sputtering surfaces of the facing target-planes. The proposed facing target type of sputtering apparatus has reflecting electrodes of electrons in the space being installed between the edge of target and the shield plate, so that the electrons are reflected to be confined in the plasma space by the reflecting electrodes. As a result, it was confirmed that the electric discharge characteristic is much improved and that a film having an excellent quality is produced under a low gas pressure and a low discharge voltage due to high rate of ionizing gases. It was also confirmed that an auxiliary magnetic field having magnetic flux parallel to the target planes is generated along around the outer peripheries of the targets and adjacent to the front surfaces of the targets for capturing sputtered electrons in the plasma space. The electric, discharge characteristic is remarkably improved in addition to the auxiliary magnetic field. Furthermore, the eroded areas of the facing targets are extensively increased, i.e., the eroded areas of facing targets are remarkably expanded to all the surfaces of targets. This implies that a uniform erosion of the entire surfaces of targets is realized. However, even in use of the facing target type of sputtering apparatus, recoiled gases and atoms sputtered from the surfaces of facing targets are transferred from the plasma space 120 between facing target planes toward all the directions on the wall of the vacuum vessel. Therefore, even in use of the proposed facing target type of sputtering apparatus, the deposition ratio on the substrate out of all the sputtered particles is limited in accordance with the configuration of the vacuum vessel 10 as illustrated in FIG. 1, and most of sputtered particles become contaminants on the surface of the vacuum chamber wall, because the porous structure and powders of contaminants contains a lot of air, H.sub.2 O and some gases.
In order to overcome the low efficiency rate of conversion of the sputtered particles to the deposited thin films on a substrate and the degradation of thin film quality due to the impingement of contaminants, the present inventor proposed a new type of facing target apparatus in Japanese Unexamined Patent Publications No.H8-162676, a box type of plasma confinement apparatus. This type of facing target type of sputtering apparatus is characterized by unique distributions of the magnetic and electric fields due to a five target arrangement in which the box-type space composed of all the five target planes has only one outlet extending toward the plane of a substrate in the vacuum vessel 10. The construction of the box-type of plasma space is composed of the first pair of facing target planes arranged to face each other so as to define a predetermined space; the second target planes are adjacent to the first pair of facing target planes arranged to cover all the side portion except the outlet facing the substrate arranged in the vacuum vessel 10. In order to confine sputtering plasma in the box type of plasma space, a pair of magnetic field generating means composed of a pair of permanent magnets so as to face N and S-pole generate magnetic flux circulating perpendicularly the outside space of the first facing targets which defines facing target mode in combination with electric fields perpendicular to target planes in plasma space. The pair of magnetic field generating means generate conventional magnetron mode which is composed of closed magnetic flux from the pole of magnets in the vicinity of the outside area of the pair of target planes in addition to the facing target mode. The cathodes of all the targets are arranged so as to recoil and confine the electrons into the plasma space by the aid of both the facing target mode and the magnetron mode as aforementioned.
In this box-type of facing target sputtering apparatus, the problems of the all the facing target types of sputtering apparatus are overcome because dense plasma can be generated and confined in the space enclosed by all the five target planes, and sputtered particles generated in the plasma space are transferred from the outlet to the substrate plane in the vacuum vessel 10.
By the way, the quality of thin films deposited at higher rate by the addition of larger input power was confirmed to become degraded rapidly corresponding to a certain high deposition rat e in both the improved facing target type of sputtering apparatus and the box-type of facing target sputtering. The tendency of degrading film quality was remarkably apparent corresponding to larger input powers under the improved facing target type of sputtering apparatus proposed in Japanese Examined Patent Publications No.H4-11624 a and H5-75827 (U.S. Pat. No. 4,784,739). The limitation of the deposition rate due to the degradation of film quality is a fatal problem in the industrial application, because production cost of thin films is largely dependent on the deposition speed of the equipment in the industrial production.